During the phase of hydration of concrete or Portland Cement which is commonly known as the stage II or dormant phase (such phase following a short stage I or aluminite C3A phase in which Ettringite crystals form) the concrete mixture passes through varying consistencies which affect workability. Typically, during the first half of the approximate two hour long dormant phase, the concrete mix remains highly workable and is capable of being poured into concrete forms. Also during such phase interval, the concrete mix is capable of removal of interior air pockets and voids through agitation. During the latter portion of the first half of the dormant phase, the mix is capable of surface smoothing via floatation troweling wherein liquid eddy currents induced by over passage of a flat trowel carry a skim layer of fine grained concrete paste over any rough aggregate which may reside at the slab's surface.
During the second half (or approximate second hour) of the concrete mix's dormant phase, the mix attains a “no-slump” consistency. Upon providing a hollow 18″ cone concrete form having an open 8″ diameter base, then filling such cone with early dormant phase concrete to a depth of 12″, then allowing the concrete set for a period of time, and then upwardly removing such cone from the concrete, the upper surface of such concrete (a 4″ diameter circular face) may “slump” or deflect downwardly a distance which is dependent to the level of set of the concrete. A downward deflection of such concrete upper surface in excess of ¼″ indicates that the removal of the conical form has occurred prior to the concrete's attainment of a “no-slump” consistency. If the concrete is “no-slump” when the form is removed, it will remain as an intact casting of the cone's interior which retains at least 98% of its originally cast height. The above described concrete working steps of pouring the mix into concrete forms, agitating the concrete to remove entrained air, and liquid float troweling the surface typically must occur prior to the concrete's setting progression to a no-slump consistency.
Nevertheless, no-slump consistency concrete may be worked in a different manner by providing a no-slump mix which comprises loosened mulch (referred to in the industry as roller compactible concrete-RCC) and by drop spreading such loose mix over the ground in an even depth layer. Such drop spreading of loosened or mulched no-slump concrete may advantageously proceed in a manner similar to the action of a common asphalt paving machine upon loose granular asphalt which is thereby conveyed from a forward hopper to a lateral dispersion auger which evenly laterally disperses the granular asphalt to rearwardly drop to the ground in an even depth layer. Upon use of such asphalt paving machinery for alternative drop spreading no-slump concrete in an even depth layer over a ground surface, the initially resulting concrete layer differs markedly from a form poured layer of pre-no-slump concrete. One major difference between the two modes of slab formation relates to air entrainment levels. Drop spread no-slump concrete typically includes a large amount of entrained air pockets which, if not removed, would produce a hardened concrete slab far weaker than conventional poured concrete. Concrete poured into forms prior to its no-slump phase includes relatively few strength degrading air pockets.
The multiplicities of entrained air pockets which commonly reside within a freshly drop spread layer of no-slump concrete necessitate that means be employed to remove the layer's air pockets prior to concrete's entry into phase III hardening. Such means typically comprise mechanical compaction of the no-slump concrete layer. Such compaction step may suitably be executed through the actions of vibratory rollers, vibratory tampers, reciprocating tampers, or vibratory screeds. Where a paving machine is utilized for executing the above described no-slump concrete drop spreading step, such compacting machinery may be conveniently and advantageously mounted upon and deployed thereon immediately rearwardly from the machine's output port. Suitably, separately deployed compacting equipment may be alternatively used.
Upon mechanical compaction of a drop spread layer of no-slump concrete, and upon removal of substantially all of such layers' entrained air pockets, phase III hardening of the concrete may proceed in a normal fashion to produce concrete slab having a strength equal to or greater than that of a conventionally poured concrete slab. However, upon such compaction and hardening, further drawbacks or disadvantages relative to conventionally poured concrete slabs are experienced.
Upon such drop spreading, compaction, and hardening of a no-slump concrete slab, multiplicities of surface imperfections resulting from exposed aggregate edges and resulting from marring or lining of surfaces at tamper or roller edges commonly remain. Unlike conventional poured concrete slabs which allow for trowel actuated liquid floatation surfacing, liquid floatation surfacing of a mechanically compacted no-slump concrete slab surface is not possible. In its no-slump state, such compressed concrete layer lacks sufficient moisture for the induction of any surface level fluid currents, frustrating any effort at floatation troweling.
Compacted no-slump concrete slabs are known to be rough and unsightly, and marked difficulties have been encountered in the concrete fabrication industry in properly surfacing such slabs.
The instant inventive method solves or ameliorates the problems, defects, and deficiencies noted above by providing apparatus and method steps which re-mulch and distribute over the surface of a compacted no-slump concrete slab a portion of the concrete's paste, such re-mulching and distribution of paste creating a smooth slab top surface.